It is known to use adjunct processors in conjunction with switching systems in order to supplement and enhance the telecommunications services provided by the switching systems. For example, an AT&T AUDIX voice mail system is often used with an AT&T Definity.RTM. PBX to provide users with voice messaging capabilities. In order to work together, the switching system and the adjunct processor must be able to communicate with each other. This is commonly accomplished through a singular digital control data link that interconnects the switching system control processor with the adjunct processor. An example of such a link is the AT&T DCIU link. As with the DCIU link, many vendors' switching systems use proprietary communications protocols on the singular digital control data link, thereby making it extremely difficult or even impossible to use other vendors' adjunct processors with their switching systems.
Various arrangements have been proposed to overcome this difficulty and to facilitate integration of different vendors' switching systems and adjunct processors. These proposals commonly rely on ignoring the singular digital control data communication link and its protocol, and obtaining the required communication capability by other means, through alternative connections to the switching system. A representative arrangement of this type is disclosed in U.S. Pat. No. 4,873,718. It discloses a device for integrating one vendor's adjunct voice mail system with another vendor's PBX. The integration device is the other vendor's feature telephone set equipped with additional circuitry which monitors the display, indicators, and function keys of the feature telephone set. The voice mail system is connected to a set of analog telephone lines of the PBX, and to the monitoring circuit. The telephone set is connected to a digital telephone line of the PBX, which line's protocol it naturally understands. Operation of the integration device is, in part, as follows. When an incoming call is to be handled by the voice mail system, the PBX first connects the call to the telephone set. In response, the monitoring circuit identifies--via inspection of an I/O device, i.e., an LCD display--and reserves a free line appearance on the telephone set. The telephone set answers the call and obtains requisite information about it from the PBX, over the digital telephone line which connects them. This information is captured by the monitoring circuit. The monitoring circuit then causes the telephone set to command the PBX to "park" the call on the free appearance that it had identified, and also passes the captured call information on to the voice mail system, including the phone number of the appearance where the call is parked. The voice mail system then selects a free analog telephone line and dials a connect feature code to access the parked call, and the involvement of the integration device in handling the call ceases.
A disadvantage of this approach is that, in having to interface to the feature telephone set as that set's display, indicators, and function keys do, it requires numerous wire connections to the feature telephone set, which potentially adversely affects its reliability. Another possible disadvantage is that it requires a feature telephone set, which may unduly add to the cost of the integration device relative to a custom implementation. A further disadvantage of this approach is that, by merely parking a call by action of the integration device, it requires the voice message system to take an active part in establishing a connection to the parked call. This is functionality which is not commonly found in voice mail systems, and hence requires that the voice mail system be modified before it can be integrated to a PBX by means of this approach.
Another disadvantage is that the aforementioned implementation of the integration device is not involved in handling the call during the call and at call completion, and therefore the voice mail system must rely on its own capabilities to detect call status such as call completion. Some PBXs provide a current level fluctuation on an analog telephone line to signal that the party on the other end of the call has hung up, and this fluctuation may be detected by the voice mail system. But other PBXs do not provide this signal, causing the voice mail system, or an adjunct thereto, to rely on return of dial tone by the PBX as an indication that the call has been completed. Detection of dial tone conventionally requires the use of additional circuitry, either as part of the voice mail system itself or as part of an adjunct device to the voice mail system. Furthermore, return of dial tone is typically delayed for some seconds after call completion, during which time the voice mail system treats the call as ongoing. This results in inefficiency, waste of voice mail system and PBX resources, and annoyance to the call message recipient if this silent portion of the message is not deleted or if the subsequent dial tone is recorded as part of the message.